I think there is lots of good advice in the comments so far about letting your uncle know that you love him, doing small things to keep in contact with him, being specific about why you're grateful for having him in your life, etc.

I also believe, based on my own experiences, that sometimes the most loving thing we can do is to tell people who matter to us "it's okay, I understand, you don't need to hang on for me." I don't know enough about your uncle's specific condition if that message is what he needs right now, but I guess I want to just gently remind you that words like this are possibility.

*Gentle hugs* I'm so sorry for how hard this must be.